Principles of Toxicology - University of FloridaBiliary excretion Plasma circulation Renal excretion...

Andrew S. Kane, Ph.D.Environmental Health Program

College of Public Health & Health ProfessionsKANE@UFL.EDU

Principles of ToxicologyThe Study of Poisons

WATER BIOLOGYWATER BIOLOGYPHC 6937; Section 4858PHC 6937; Section 4858

“The problem with toxicology is not the practicing toxicologists, but chemists who can detect, precisely, toxicologically insignificant amounts of chemicals” Rene Truhaut, University of Paris (1909-1994)

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Sources of Environmental Chemicals

Air EmissionsIndustrial ProcessesIncineratorsGasoline and diesel exhaustSpraying of agricultural chemicals

Water DischargesIndustrial effluentsSewage effluent

Non-Point SourcesSurface run-off from roads and agricultural landLeachate from dump-sitesAccidental spills

Household Chemical Use

• Aquatic animals as environmental sentinels

Toxicology…………• Is the study of the harmful effects of chemicals and physical

agents on living organisms

• Examines adverse effects ranging from acute to long-term

• Is used to assess the probability of hazards caused by adverseeffects

• Is used to predict effects on individuals, populations andecosystems

Descriptive Toxicology: The science of toxicity testing to provideinformation for safety evaluation and regulatory requirements.

Mechanistic Toxicology: Identification and understanding cellular,biochemical & molecular basis by which chemicals exert toxic effects.

Regulatory Toxicology: Determination of risk based on descriptiveand mechanistic studies, and developing safety regulations.

Federal agencies: FDA (FDCA- Federal Food, Drug & Cosmetic Act)

EPA (FIFRA-Federal Insecticide, Fungicide and Rodenticide Act)

EPA (TSCA-Toxic Substance Control Act)

EPA (CERCLA- Comprehensive Env Response, Compensation, &Liability Act); Superfund

DOL (OSHA-Occupational Safety and Health Administration)

An interdisciplinary field…

Clinical Toxicology: Diagnosis and treatment of poisoning;evaluation of methods of detection and intoxication, mechanism ofaction in humans (human tox, pharmaceutical tox) and animals(veterinary tox). Integrates toxicology, clinical medicine, clinicalbiochemistry/pharmacology.

Occupational Toxicology: Combines occupational medicine andoccupational hygeine.

Environmental Toxicology: Integrates toxicology with sub-disciplines such as ecology, wildlife and aquatic biology,environmental chemistry.

An interdisciplinary field…

Approach

Classical Toxicology Mechanistic Toxicology

ADME

Target Organ Effects

Mechanisms of CellInjury and Cell Death

In vitro/In vivo ModelsWhole Animal Studies

Gene Environment Interactions

Susceptible PopulationsSusceptible Populations

Diagram of quantaldose-responserelationships

Approximate acute LD50s for selected chemical agents

Relative Toxicity

Comparison of dose-response relationship for twodifferent chemicals plotted on a log dose-probit scale

“All substances are poisons: there is none which is not a poison. The right dose differentiates a poison and a remedy.” Paracelsus (1493-1541)

Dose

Effe

ct

Absorption

Distribution to tissues

Metabolism

Excretion

Classical Toxicology

Dose: Amount of chemical an organism isexposed to per unit of body weight(mg/kg b.wt)

Exposure: Concentration of a chemical in eitherthe air or water through whichthe exposure occurs

Dose vs Exposure

Concentrations in liquids or solids:ppt = parts per thousand (g/L; ‰; PSU); easily confusedppm = parts per million (µg/mL = mg/L or µg/g = mg/kg)ppb = parts per billion (ng/mL = µg/L or ng/g = µg/kg)

Concentrations in air:mg vapor/m3 =molecular weight (ppm)/24.45ppm = ug/m3

Exposure concentrations

Primary Routes of Exposure

Gastrointestinal

Respiratory

Dermal (skin)

There are tremendous differences in the absorption of compounds depending on the route of exposure due to physiological differences between these organs.

Great differences between various species.

Metabolism

Metabolites: conversion products of substances,often mediated by enzyme reactions.

Bioactivation (activation): production ofmetabolites that are more toxic than the parentsubstance.

Detoxication: production of metabolites that areless toxic than the parent substance.

XenobioticsXenobiotics

Accumulation(storage in body fat, bone)

Phase I metabolism(bioactivation or inactivation)oxidation, reduction, hydrolysis

Phase II metabolism(bioinactivation) conjugation

Extracellular mobilization

Biliary excretion

Plasma circulation

Renal excretion

highly lipophilicmetabolically stable

lipophilic polar hydrophilic

polar

hydrophilic

Overview of possible types of phase I biotransformation reactions

Oxidation reactions:Loss of electrons, oftenaddition of O to replace H

Reduction reactions:Gain of electrons, oftenaddition of H to replace O

Hydrolysis reactions:Water interacts withsubstrate such that O2makes bond

Phase II Conjugation Reactions

Routes of Elimination

Biliary

Renal

Fecal

Respiratory

Pharmacokinetic Parameters

Volume of distribution: Vd = Total dose/[blood]

The apparent volume of distribution (Vd) is the volume of fluidwhich the drug would occupy if it were evenly distributedthrough that volume at the concentration measured in theplasma (central compartment).

Vd is a convenient method for describing how well a drug isremoved from the plasma and distributed to the tissues.However, it doesn't provide any specific information aboutwhere the drug is or whether it is concentrated in a particularorgan. A large volume of distribution implies wide distribution, orextensive tissue binding, or both. Conversely, ionized drugs thatare trapped in plasma, will have small volumes of distribution.

Pharmacokinetic Parameters

Half-life:

The half-life (t1/2) is the time taken for the xenobioticconcentration to decline by 50%. It is related to therate constant by the following:

t1/2 = 0.693/ kel

50 % of the xenobiotic is lost in 1.00 half-life90 % of the xenobiotic is lost in 3.32 half-lives95 % of the xenobiotic is lost in 4.32 half-lives99 % of the xenobiotic is lost in 6.64 half-lives

Pharmacokinetic Parameters

Elimination rate constant: kel = 2.303 x slope

The serum level curve observedfrom a xenobiotic eliminated by afirst order process.

A plot of this same data using alog scale on the y-axis results ina straight line.

Pharmacokinetic Parameters

Octanol Water PartitionCoefficient (Kow)

•Ratio of the concentrationof a chemical in octanol andin water at equilibrium andat a specified temperature.

•Predict solubility

•Predict bioaccumulation

Bioaccumulation

• Accumulation of substances, such as pesticides or other organicchemicals in an organism or part of an organism.

• Biological sequestering through respiration, diet, epidermal (skin) contact.

• Results in the organism having a higher concentration of the substancethan the concentration in the surrounding environment.

• Amount depends on the rate of uptake, the mode of uptake, how quicklythe substance is eliminated, transformation of the substance, the lipidcontent of the organism, the Kow of the substance, and environmentalfactors, and other biological and physical factors.

• General rule: the more hydrophobic a substance is the more likely it is tobioaccumulate in organisms. Exceptions (e.g. methylmercury).

• Bioconcentration refers only to the uptake of substances into the organismfrom water alone. Bioaccumlation is the more general term because itincludes all means of uptake into the organism.

Relationship between dose and concentration at the target siteunder different conditions of dose frequency and elimination rate

Pharmacokinetic Parameters

Haber’s Law

For many compounds…

The toxic effect of a substance is determined bythe product of the concentration and theduration of the exposure

Dose-response relationship for representativeessential substances, such as vitamins or trace elements (e.g., Cr, Co, Se)

Hormesis

Effective, toxic and lethal dosages

• Acute effects do not predict chronic effects

• Doses causing chronic effects may not cause acute orsub-acute effects

• In human and veterinary arenas chronic effects of achemical exposure may manifest themselves as acommon disease and go unnoticed

• SARs and Kow predictors

Acute vs Chronic Toxicity

Chemical InteractionsAdditive: 2+3=5 (2 OPs - cholinesterase inhibition)

Synergistic: 2+2=20 (CCl4 + EtOH)

Potentiation: 0+2=10 (isopropanol + CCl4)

Antagonism: 4+6=8; 4+0=1•Functional antagonism: 2 chemicals counterbalance each other byproducing opposite effects on the same physiologic function(eg epinephrine + diazepam).

•Chemical antagonism (inactivation): chemical rxn between 2compounds that produces a less toxic product (eg chelators and metals).

•Dispositional antagonism: alters A,D,M or E to that conc or duration attarget site is diminished (eg ipacac, charcoal, diuretics, SKF-525A orpiperonyl butoxide).

•Receptor anatagonists (blockers): clinical trtmt by competitive bindingto same receptor (eg atropine and OPs to block cholinesterase receptors;tamoxifen as an anti-estrogen to lower risk of breast cancer).

Methods for Measuring the Acute Toxicity ofEffluents and Receiving Waters to Freshwaterand Marine Organisms

Methods for Measuring the Acute Toxicityof Effluents and Receiving Waters toFreshwater and Marine Organisms

Freshwater:Ceriodaphnia dubia (daphnid)Daphnia pulex and D. magna (daphnids)Pimephales promelas (fathead minnow)Oncorhyncus mykiss (rainbow trout)

Estuarine & Marine:Mysidopsis bahia (mysid)Cyprinodon variegatus (daphnids)Menidia beryllina, M. menidia & M. peninsulae (inland, Atlantic & tidewater silversides)

Methods for Measuring the Acute Toxicityof Effluents and Receiving Waters toFreshwater and Marine Organisms

TemperatureLight qualityLight intensityPhotoperiodTest chamber sizeTest solution volumeRenewal of test solutionsDensity of test organisms

AerationDilution waterNumber of replicatesAge of test organismsTest concentrationsDilution factorTest durationEndpoints